1. Field of the Invention
The present invention relates to a technique for transferring a substrate to and accurately positioning the same at a predetermined position on a substrate holding device included in a processing unit for processing a substrate held on a substrate holding device by a coating process for coating the substrate with, for example, a resist solution and a developing process.
2. Description of the Related Art
A photoresist process, which is one of semiconductor device fabricating processes, includes the steps of forming a thin resist film on a surface of, for example, a semiconductor wafer (hereinafter, referred to simply as “wafer”), exposing the resist film to light through a mask of a predetermined pattern, and developing the exposed resist film to obtain a predetermined mask pattern. Generally, these steps of the photoresist process are carried out by a processing system built by combining a coating and developing system and an exposure system.
To achieve high throughput and to minimize footprint, the substrate processing system includes a necessary number of processing units respectively including processing devices respectively for processing a substrate by different processes, such as a coating process, a developing process and a heating and cooling process, and stacked in layers, and substrate carrying means for carrying a substrate to those processing units.
Generally, a wet processing unit for carrying out coating and developing processes, among those processing units, carries out a spin-coating process by holding a wafer substantially horizontally by a spin chuck, namely, a substrate holding device, pouring a process solution, such as a resist solution or a developer, onto a substantially central part of the wafer, and rotating the spin chuck.
The wafer is required to be transferred to the spin chuck and to be accurately positioned at a predetermined position on the spin chuck to process the wafer accurately by the wet processing unit. For example, a coating unit that uses a resist solution, namely, a process solution, spreads the resist solution on a surface of a wafer by spin-coating, and executes an edge rinsing process to remove an edge part of a resist film by pouring a cleaning solution onto a peripheral part of the rotating wafer. If the wafer is eccentric with respect to the rotation axis of the spin chuck during the edge rinsing process, the resist film is not removed from some areas of the a peripheral part of the wafer and the wafer is removed excessively from some other areas of the peripheral part. Consequently, a defective resist film is formed on the wafer.
A teaching procedure is executed before starting the substrate processing system to teach a position on the spin chuck at which a wafer is to be placed to the substrate carrying means of the substrate processing system. A teaching procedure disclosed in JP-A 2005-19963 (Paragraphs 0035 to 0037, FIGS. 8 and 9) places a wafer marked with a printed, small circular mark on a spin chuck, images of the circular mark are obtained by CCD camera disposed above the wafer while the spin chuck is intermittently turn at angular intervals of 90°, determines the rotation center of the spin chuck on the basis of displacements of the circular mark, and teaches the carrying means placing a wafer on the spin chuck with the center of the wafer coincided with the rotation center of the spin chuck.